Auxiliary Power Unit, abbreviated as APU, is a small turbine engine mounted on the tail of an aircraft. Its main function is to supply power and a gas source, and some APUs are capable of providing additive thrust to the aircraft. Specifically, before taking off from the ground, an aircraft may rely on power and gas supply from the APU to start the main engine, rather than the ground power and gas source vehicles. While on the ground, the APU also supplies power and compressed air to ensure lighting and air-conditioning in the cabin and cockpit. During take-off of an aircraft, the APU can serve as a backup power source. After the aircraft is landed, lighting and air-conditioning of the aircraft are still maintained by power from the APU. The functions of APU influences the flight stability of the aircraft, which directly affects flight cost and quality of service of the aircraft.
APU lubricant cooler is an important component in APU, and the operation of APU is directly impacted by the performance of the lubricant cooler. Poor operation of lubricant cooler is likely to result in shut-down of APU, or even grounding of the aircraft. Once grounding occurs, the cost for consequent maintenance and repair is very expensive. At present, there are two methods for maintenance of the APU lubricant cooler. One is weekly inspection, that is, measuring and calculating the temperature of the lubricant weekly, and checking whether the temperature exceeds the limit. However, due to omission, uninspected case of exceed the temperature limit may happen, and thus the shut-down of APU cannot be avoided absolutely. The other is periodical inspection of lubricant cooler according to the reliable statistical data. Although the latter can effectively avoid the shut-down of APU caused by the performance degradation of the lubricant cooler, replacement or repair in fixed period cost more air materials, and hence it is not beneficial for reducing the maintenance cost.
Therefore, an effective method for monitoring the performance of APU lubricant cooler is desired in the art so that the system may issue an alarm in case of dysfunction of lubricant cooler. Thus, omitted inspection or over maintenance of APU lubricant cooler will not happen.